ic6b01922_si_005.cif (1.46 MB)
Elucidating the Mechanism of Uranium Mediated Diazene NN Bond Cleavage
dataset
posted on 2016-11-02, 12:34 authored by John J. Kiernicki, Robert
F. Higgins, Steven J. Kraft, Matthias Zeller, Matthew P. Shores, Suzanne C. BartInvestigation
into the reactivity of reduced uranium species toward diazenes has
revealed key intermediates in the four-electron cleavage of azobenzene.
Trivalent Tp*2U(CH2Ph) (1a) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) and Tp*2U(2,2′-bpy) (1b) both perform the two-electron
reduction of diazenes affording η2-hydrazido complexes
Tp*2U(AzBz) (2-AzBz) (AzBz = azobenzene) and
Tp*2U(BCC) (2-BCC) (BCC = benzo[c]cinnoline) in contrast to precursors of the bis(Cp*) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide)
ligand framework. The four-electron cleavage of diazenes to give trans-bis(imido) species was possible by using Cp*U(MesPDIMe)(THF) (3) (MesPDIMe = 2,6-((Mes)NCMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), which is supported by a highly
reduced trianionic chelate that undergoes electron transfer. This
proceeds via concerted addition at a single uranium center supported
by both a crossover experiment and through addition of an asymmetrically
substituted diazene, Ph-NN-Tol. Further investigation of 3 and its substituted analogue, Cp*U(tBu-MesPDIMe)(THF) (3-tBu) (tBu-MesPDIMe = 2,6-((Mes)NCMe)2-p-C(CH3)3-C5H2N), with benzo[c]cinnoline, revealed
that the four-electron cleavage occurs first by a single electron
reduction of the diazene with the redox chemistry performed solely
at the redox-active pyridine(diimine) to form dimeric [Cp*U(BCC)(MesHPDIMe)]2 (5) and Cp*U(BCC)(tBu-MesPDIMe) (6). While a transient pyridine(diimine) triplet diradical
in the formation of 5 results in H atom abstraction and p-pyridine coupling, the tert-butyl moiety
in 6 allows for electronic rearrangement to occur, precluding
deleterious pyridine-radical coupling. The monomeric analogue of 5, Cp*U(BCC)(MesPDIMe) (7), was synthesized via salt metathesis from Cp*UI(MesPDIMe) (3-I). All complexes have been characterized
by 1H NMR and electronic absorption spectroscopies, X-ray
diffraction, and, where pertinent, EPR spectroscopy. Further, the
electronic structures of 3-I, 5, and 7 have been investigated by SQUID magnetometry.